Syndecan-1 is one of a family of heparan sulfate proteoglycans that function as receptors in extracellular matrix adhesion. Although syndecan-1 is reportedly not expressed on endothelial cells lining mature vessels, we have found that it is expressed on activated endothelial cells undergoing tumor-induced angiogenesis and propose that it regulates the activity of the alpha-v-beta3 and alpha-v-beta5 integrins at this site;these two integrins are integral to the pathways by which FGF and VEGF induce angiogenesis, particularly providing protection against apoptosis. Our recent work has identified a novel mechanism in which syndecan-1 regulates the activity of the alpha-v-beta3 and alpha-v-beta5 integrins. Our preliminary findings suggest that silencing of syndecan-1 expression, or competition with the syndecan-1 ectodomain expressed in bacteria, will block the activation of these two integrins in endothelial cells. We propose to localize the site(s) responsible for regulating these integrin (s) and to isolate peptides representing the active sites in syndecan-1 that can be used competitively in angiogenesis assays. We also plan to molecularly characterize the regulatory complex that encompasses the syndecan and each integrin. Since syndecan-1 appears upregulated in vascular endothelium undergoing tumor-induced angiogenesis, we will explore what regulates this expression during the angiogenesis program, comparing stimulation by FGF and VEGF, and correlating syndecan-1 expression with the Hox D3, B3 and DIG master regulatory genes. These assays will also measure the two different downstream pathways of protection against apoptosis activated by either FGF or VEGF, as they have been shown to depend on the alpha-v-beta3 and alpha-v-beta5 integrins, respectively, and determine whether both rely on the syndecan-1-mediated regulation of these integrins. Finally, we will use this information, and the competitive peptides that we hope to derive, to disrupt the syndecan-1 regulation in vivo, using a mouse cornea angiogenesis assay, and angiogenesis induced by mouse tumors. These studies will provide new insights into how syndecan-1 functions during angiogenesis, and will hopefully lead to new drugs that block the deleterious angiogenesis that occurs in diseases such as cancer, endometriosis, diabetic retinopathy and others.